Liposome formation is a natural result of the amphipathic nature of the molecules of which they are comprised. Amphipathic molecules are those molecules with distinct regions of the molecule having hydrophilic character and distinct regions of the same molecule having hydrophobic character. When dispersed in water, amphipathic molecules form three types of macro-molecular structure: micelles, hexagonal phase and lipid bilayers. The exact macromolecular structure which is formed depends on the relative sizes of the hydrophilic and hydrophobic regions of the molecule.
Micelle formation is favored when the cross sectional area of the hydrophilic region of the molecule is greater than that of the hydrophobic part of the molecule. Detergents are examples of such molecules, e.g., sodium palmitate. Detergents contain a hydrocarbon chain (the hydrophobic portion of the molecule) and an ionic base (the hydrophilic portion of the molecule), and act as emulsifying agents to bind water and oil phases. That is, detergents allow oil and water to be broken into tiny droplets suspended or dispersed in water. Particular detergents 1 may be classified as anions (negatively charged at the hydrophilic portion) and may be represented, as shown in FIG. 1, as having a hydrophilic head 2 with a hydrocarbon (hydrophobic) tail 4. FIG. 2 is a representation of a micelle structure 5 formed of a number of detergent molecules due to their hydrophilic/hydrophobic character.
In the opposite conformation, i.e., when the cross sectional area of the hydrophobic region of the molecule is greater than that of the hydrophilic part of the molecule, the formation of hexagonal phase structures is favored, e.g., dimyristoyl-phosphatidylethanolamine (DMPE). FIG. 4A is a representation of a hexagonal phase structure, sometimes referred to as an inverse micelle.
For molecules in which the cross sectional area of the hydrophilic region of the molecule is slightly less than, or equal to, that of the hydrophobic part of the molecule, such as many phospholipids, the formation of bilayers is favored, e.g., dipalmitoylphosphatidylcholine (DPPC). Phospholipids are an amphipathic type of lipid which contain phosphate, that is, molecules containing one phosphate, a glycerol and one or more fatty acids. FIG. 3 is a simplified representation of a phospholipid molecule 6, including a hydrophilic head 8 (i.e., the phosphate and glycerol) and a hydrophobic tail 10 (i.e., the one or more fatty acids). FIG. 4 is a representation of a phospholipid bylayer 12, where the hydrophobic regions 14 of the phospholipid molecules are caused to turn inward due to the aqueous environment, and the hydrophilic portions 16 face outward. These bilayers are two dimensional sheets in which all of the hydrophobic portions, e.g., acyl side chains, are shielded from interaction with water except those at the ends of the sheet. An energetically unfavorable interaction of the acyl chains with water results in the folding of the bilayers to form three-dimensional, vesicles. These vesicles are referred to as "liposomes".
Liposomes may be formed as a single bilayer enclosing a single aqueous space (small unilamellar vesicles; SUVS) or may be composed of concentric bilayers with many aqueous spaces alternating with the bilayers (multilamellar vesicles; MLVS). Liposomes can be used to encapsulate both hydrophobic and hydrophilic materials. Hydrophobic payloads are typically partitioned within the bilayers whereas hydrophilic payloads are typically trapped within the aqueous compartments. The advantages of using liposomes as a carrier/encapsulation system is that they are stable and can protect their payload from degradation, e.g., by oxygen, digestive enzymes, etc.
For example, U.S. Pat. No. 3,957,971, issued May 15, 1976, discloses liposome-formed moisturizing units which are capable of moisturizing and improving flexibility, plasticity, and softness of keratinous matter, particularly mammalian skin. The liposomes within which the moisturizer is stored include a matrix of a ternary lipid mixture of lecithin, dicetyl phosphate, and a sterol, and include cavities disposed within the liposome. The cavities (lamellar space) contain an humectant, such as sodium pyroglutamate, in an aqueous medium. Moisturizing liposomes are also disclosed therein which function osmotically, serving as traps for water, which may be shared with the keratin constituents as required.
Liposomes also may be used for the timed delivery of a wide variety of materials including pharmaceuticals, cosmetics and nutrients. For example, U.S. Pat. No. 4,016,100, issued Apr. 5, 1977, discloses a method of producing a pharmaceutical composition comprised of an aqueous suspension of an active agent entrapped in a spherule of a phospholipid (liposome) The composition, or drug delivery vehicle, is prepared by dispersing a phospholipid uniformly in water to give an aqueous phospholipid dispersion, adding a medicament to the aqueous dispersion and freezing the thus-obtained aqueous dispersion to entrap the medicament in lipid spherules formed. The frozen dispersion is then thawed to realize an aqueous suspension of spherules having diameters of less than 5.0 microns. The timed release of an active agent is directly related to the amount of active agent trapped in the liposomes. The greater the amount of active agent, the longer the release process lasts.
A goal of the liposome research has been the development of a liposomal delivery system that would deliver its payload not over time as in the '100 patent mentioned above, but on cue, i.e., a controlled release, for example, in a mammalian body. For example, a delivery system that delivers its payload when applied to the skin or when arriving at a tumor. A bulk of the research has been based on admixtures of liposomes and other biological macromolecules such as antibodies and lecithins. Various degrees of success have been achieved with these systems but none have produced a liposome that will release its payload, or not, depending on the prevailing conditions. The invention described herein is just such a liposome: the degree of, payload encapsulation may be altered by changes in pH and/or ionic strength of the surrounding medium thereby realizing a triggered delivery system in a form of a liposome.
Vitamin A derivatives are desirable ingredients in cosmetic formulations. Retinol (vitamin A) is an endogenous compound which occurs naturally in the human body and is essential for normal epithelial cell differentiation. Natural and synthetic vitamin A derivatives have been used extensively in the treatment of a variety of skin disorders and have been used as skin repair or renewal agents. Retinoic acid has been employed to treat a variety of skin conditions, e.g., acne, wrinkles, psoriasis, age spots and discoloration. See e.g., Vahlquist, A. et al., J. Invest. Dermatol., Vol. 94, Holland D. B. and Cunliffe, W. J. (1990), pp. 496-498; Ellis, C. N. et al., "Pharmacology of Retinols in Skin", Vasel, Karger, Vol. 3, (1989), pp. 249-252; Lowe, N.J. et al., "Pharmacology of Retinols in Skin", Vol. 3, (1989), pp. 240-248; PCT Patent Application No. WO 93/19743.
It is believed that the use of retinol and retinyl esters would be preferred over the use of retinoic acid in the treatment of skin due to the fact that retinol is both an endogenous compound found in the body and are considered much safer than retinoic acid.
Retinol and retinyl esters are known for their beneficial effects on the skin, in particular in topical application. Retinol and retinyl esters have for a long time been used in the treatment of acne. However, it is in the field of repair of damage caused either by age or by over-exposure to the sun that retinol has proven to be extremely active. Thus, the effects of retinol retinyl esters on cell differentiation make it possible to envisage their use thereof for effectively combating the appearance of wrinkles and fine lines, and for combating dryness, roughness and/or stiffness of the skin. In addition, they are active as antioxidants in the regeneration of tissues. Repeated application of cosmetic compositions containing retinol and/or retinyl esters have enabled wrinkles to be removed, the skin to be rendered smooth and small cracks in the epidermis to be repaired.
On account of these beneficial effects, it has for a very long time been sought to formulate retinol and retinyl esters in cosmetically acceptable compositions in a form which is stable over time at room temperature. However, retinol and retinyl esters have been shown to be unstable when stored over time and are thus difficult to formulate into cosmetic compositions. Particularly, retinol and retinyl esters are highly sensitive to elevated temperatures and result in significant color changes upon degradation.
Thus, although retinol and retinyl esters are considered safer to use than retinoic acid, they are less effective than retinoic acid at providing skin benefits due to their decreased stability which leads to inactivation. It would thus be highly desirable to stabilize the unstable vitamin A derivatives, retinol and/or retinyl esters, for use in the treatment of skin.
A number of methods have been adopted to stabilize the breakdown of vitamin A.
U.S. Pat. No. 5,738,858 discloses skin care compositions containing fatty hydroxyethyl imidazoline surfactants in combination with retinol and/or retinyl esters for use in stabilizing such compounds.
U.S. Pat. No. 5,756,109 discloses skin care compositions containing geranyl geraniol in combination with retinol and/or retinyl esters for use in stabilizing such compounds.
U.S. Pat. No. 5,759,556 discloses skin care compositions containing cyclic aliphatic unsaturated aldehydes, ketones alcohols or esters in combination with retinol and/or retinyl esters for use in stabilizing such compounds.
U.S. Pat. No. 5,744,148 discloses skin care compositions containing emulsions of unstable retinol and/or retinyl esters in an oil phase for use in stabilizing such compounds.
Encapsulation of retinol into liposomes has also been shown to be effective in enhancing the stability of the encapsulated vitamin A derivatives, however, none of these liposomes incorporate the preferred cationic long chain alkylammonium fatty acid salt liposomes of the invention.
U.S. Pat. No. 5,679,374 discloses the encapsulation of retinol into two different types of liposome compositions which allow for the simultaneous action of two different active agents, one of which may be retinol and its derivatives. The different liposomes used provide for penetration into different areas of the skin, i.e. surface layers and deep layers. The liposomes used are not cationic liposomes and there is no enhancement of the stability of the retinol is shown.
WO 96/31194 discloses the encapsulation of retinoids into non-phospholipid, non-ionic liposomes which provide for increased chemical stability over a long period of time. However, the liposomes used are non-ionic and do not display the characteristics of the cationic liposomes of the invention.
U.S. Pat. No. 5,192,544 discloses the encapsulation of a retinoid compound into phospholipid liposomes with the concomitant incorporation of a pyrimidine derivative which is used to enhance stabilization.
It has now surprisingly been found that unstable vitamin A derivatives, when encapsulated in liposomes formulated with long chain alkylammonium fatty acid salts as described in U.S. Pat. No. 5,874,105, issued Feb. 23, 1999, show a significant improvement in long term stability.